Corrosion – Definition, Types, Prevention, Examples

Electrochemistry is a discipline of physical chemistry concerned with the link between electrical potential as a quantifiable and quantitative phenomenon and recognisable chemical change, with electrical potential as a result of a specific chemical change or vice versa. Electrons move between electrodes through an electronically conducting phase (usually, but not always, an external electrical circuit like as in electroless plating), which is separated by an ionically conducting and electrically insulating electrolyte (or ionic species in a solution).

Corrosion

Corrosion is described as the chemical and electrochemical destruction of metals or alloys by the surrounding damp and humid environment. Corrosion is an oxidation process since it occurs in the presence of oxygen. Almost all metals corrode, however the rate of corrosion varies depending on the metal.

Metals generate stable compounds such as oxides, hydroxides, and sulphides as a result of corrosion, which is a spontaneous and irreversible process.

Factors Influencing Corrosion

1. Position of metals in Electrochemical series – In corrosion, the electrochemical series is extremely important. Reactive metals lose electrons more quickly and corrode more quickly. Iron, for example, is quickly oxidised, but gold is not because gold is less reactive than iron.
2. Impurities in metals – In general, the presence of contaminants in metals accelerates corrosion because these impurities operate as microscopic electrochemical cell that causes corrosion.
3. Presence of electrolytes – Because they carry ions, dissolved salt in water acts as an electrolyte. Corrosion is accelerated when electrolytes are present in water.
4. The concentration of Oxygen – Corrosion is accelerated by an increase in oxygen content. An anode is a location with less oxygen concentration, whereas a cathode is a region with higher oxygen concentration. Corrosion happens as a result.
5. Humidity in weather – When the weather is humid, the presence of moisture with a high temperature accelerates corrosion because ions gain energy and begin moving quicker in a higher temperature, causing them to collide more frequently.

Types of Corrosion

1. Uniform Corrosion- Corrosion of this sort is fairly prevalent. This kind of corrosion results in a homogeneous loss of metal throughout a whole surface. Rusting, tarnishing of silver, and so forth are examples.
2. Pitting Corrosion- Pitting corrosion is a type of localised corrosion that damages the metal surface, resulting in patches or pits. In neutral or acid solutions containing halides, such as saltwater containing NaCl, pitting corrosion can be observed in stainless steels.
3. Crevice Corrosion- Crevice corrosion is caused by a stagnant solution in crevices around the edges of metal surfaces, such as nuts and rivet heads. It can happen when two metals collide or when a metal collides with a non-metal. Corrosion on metal or non-metal gaskets, couplings, and joints, for example.
4. Galvanic Corrosion- When two dissimilar materials are connected and brought into electrical contact with a corrosive electrolyte, galvanic corrosion occurs. In galvanised iron, for example, a sheet of iron or steel is coated with zinc.
5. Stress Corrosion Cracking- It’s a form of corrosion in which fractures appear on the metal’s surface as a result of the combined effects of tensile strength and the corrosive environment. Stress Corrosion Cracking (SCS) is a kind of corrosion that occurs in aviation components.
6. Intergranular Corrosion- It’s a sort of corrosion that happens around the grain boundaries of a metal or alloy that contains impurities, rather than inside the metal or alloy. Die-cast zinc alloys containing aluminium exposed to steam in a maritime environment display intergranular corrosion.
7. Dealloying- It’s a rare sort of corrosion that only affects particular metals, such as copper alloys and grey cast iron. The alloy metal loses its reactive element and creates the more stable corrosion-resistant element in a porous condition during the dealloying process. De-nickelification, de-zincification, and graphitic corrosion are all instances of dealloying.
8. Erosion Corrosion- Erosion Corrosion is a mechanically induced breakdown of a metal surface. In a corrosive environment, it is caused by the relative mobility of fluid particles and solid surfaces. Burrs left at the cut tube ends of pipelines, for example, might disrupt smooth water flow, causing localised turbulence and high flow velocities, resulting in erosion-corrosion.

Various Examples of Corrosion

Metals with a higher reactivity series, such as iron, copper, and zinc, corrode more quickly. Corrosion can manifest itself in a variety of ways.

Rusting of Iron

Rusting is the process of iron metal corroding in the presence of air and water, resulting in a reddish-brown surface. It occurs when iron oxidises in the presence of oxygen and water. Rust is a novel substance with the chemical formula Fe₂O₃.xH₂O that is the hydrated form of Iron (III) oxide (where x is variable).

Though iron rusting is a natural occurrence, the process closely mimics an electrochemical cell reaction.

The anode is formed when iron loses electrons and becomes Fe²⁺.

At Cathode: 

H⁺+e–→H

4H+O₂→2H₂O

4H⁺+O₂+4e^–→2H₂O

The ferrous oxides are further oxidised to ferric ions, which are then combined with H₂O to produce a complex.

Oxidation Half-reaction

Fe(s)→Fe₂+(aq)+2e^–×2(E^∘=–0.44V)

Reduction Half-reaction

4H⁺+O₂+4e^–→2H₂O(E^∘=1.23V)

Overall Cell reaction

E^∘_cell = E^∘_red – E^∘_oxd

E^∘_cell = 1.23–(–0.44)V

E^∘_cell = 1.67V

2Fe(s)+4H⁺+O₂→2Fe²⁺(aq)+2H₂O(E^∘=1.67V)

2Fe₃++3O₂→2Fe₂O₃

Fe₂O₃+xH₂O→Fe₂O₃.xH₂O(rust)

Corrosion of Copper

When exposed to the environment, copper materials or copper alloys corrode. When copper oxidises, the shiny copper surface tarnishes, turning dark brown or black, then green. Copper corrodes as a result of this leaching.

The blue-green colour of the “Statue of Liberty” is an example of copper metal corrosion. It was not painted the same colour as it is now when it was built. Rather, it was brown in colour. Copper, on the other hand, became oxidised over time and interacted with sulphur trioxide, carbon dioxide, and water in the atmosphere to generate a new chemical that gave it the distinctive blue-green patina.

The following is the chemical equation involved in copper metal corrosion:

Tarnishing of Silver

Chemical interaction between the silver and sulphur-containing compounds in the air causes the silver to tarnish. Silver sulphide is formed when silver is mixed with sulphur. Tarnishing is the process of the silver metal becoming black due to a covering of silver sulphide.

The following is the chemical equation for the tarnishing of silver metal:

2Ag+H₂S → Ag₂S+H₂

Methods to Prevent Corrosion

Corrosion is an unwelcome phenomenon that must be avoided since it causes significant damage to metallic constructions such as bridges, sculptures, monuments, metal utensils, and so on.

All corrosion prevention methods are based on the same basic concept: cutting off the metals’ water and air supply. The following are some of the corrosion prevention measures:

1. Painting- Painting iron items, such as gates and rails, prevents metal from being exposed to air and water. Painting metallic items protect them from corrosion.
2. Oiling and Greasing- Oiling and greasing, like painting, provide a protective coating on the metal surface that protects it from corrosion.
3. Galvanisation- Galvanisation is the process of coating iron and steel things with a thin layer of zinc to protect them. The galvanised products are protected against corrosion or rusting even after the zinc coating has been broken.
4. Alloying- Alloys are made up of a homogeneous combination of metals and non-metals. We may achieve the required characteristics of metals by alloying them. Iron, for example, is a very useful metal, yet it rusts fast. As a result, when this iron is combined with Nickel and Chromium, stainless steel is created. Stainless steel is now widely utilised in the manufacture of kitchenware.

Sample Questions

Question 1: What is Corrosion? 

Answer

Corrosion is the chemical and electrochemical deterioration of metals or alloys caused by the surrounding damp and humid environment. Corrosion is an oxidation process since it occurs in the presence of oxygen.

Question 2: What are the ways to prevent corrosion?

Answer

• Painting, oiling, and greasing iron products prevents the metal from being exposed to air and water. Corrosion is therefore avoided.
• Galvanisation is the process of coating iron and steel things with a thin layer of zinc to protect them. The galvanised products are protected against corrosion or rusting even after the zinc coating has been broken.
• We can achieve the required qualities of metals and prevent corrosion by alloying them.

Question 3: What are Factors Affecting Corrosion?

Answer

The position of metals in the electrochemical series, impurities in metals, presence of electrolytes, oxygen concentration, weather humidity, and other environmental factors such as high temperature, presence of various gases in the atmosphere that cause corrosion, etc. are all factors that affect corrosion.

Question 4: What are Types of Corrosion?

Answer

The various types of corrosion include Uniform Corrosion, Pitting Corrosion, Crevice Corrosion, Galvanic corrosion, Stress Corrosion Cracking, Intergranular Corrosion, Dealloying, Erosion Corrosion.

Question 5: What is the tarnishing of silver?

Answer

The silver tarnishes due to a chemical reaction between the silver and sulphur-containing chemicals in the air. When silver is combined with sulphur, silver sulphide is created. Tarnishing occurs when a layer of silver sulphide coats the silver metal, turning it black.

2Ag+H₂S → Ag₂S+H₂

Question 6: What is rusting of iron?

Answer

In the presence of air and water, iron corrodes, generating iron oxide and rust.

Fe₂O₃+xH₂O→Fe₂O₃.xH₂O

Question 7: What is Crevice Corrosion?

Answer

A stagnant solution in gaps around the edges of metal surfaces, such as nuts and rivet heads, causes crevice corrosion. When two metals clash or when a metal collides with a non-metal, this can happen. For example, corrosion on metal or non-metal gaskets, couplings, and joints.